(CNN) -- Tar balls believed to be from the undersea gusher in the Gulf of Mexico have reached the shores of Louisiana's Lake Pontchartrain, a foundation that monitors the watershed reported Monday.
The area affected covers a stretch of up to five miles near the city of Slidell, northeast of New Orleans, said Anne Rheams, executive director of the Lake Pontchartrain Basin Foundation. She estimated the amount of oil that has reached the lake at less than 100 barrels, with no hydrocarbon smell.
"They are about the size of a silver dollar, maybe a little bigger, kind of dispersed in long intervals. It's not as dense as it could be, so we're thankful for that," Rheams said.
Cleanup crews had collected more than a half-ton of the tar balls -- 1,020 pounds -- by Monday evening, and the oil will be tested to confirm it came from the undersea well that has been spewing into the Gulf since April, the Joint Information Center in Houma said Monday night. But Rheams told CNN, "They've got to be from the spill."
Tar balls had previously been spotted in Rigolets Pass, which connects the lake with Mississippi Sound. Officials in Orleans and St. Tammany parishes have been using heavy booms, barges and skimmers to defend Pontchartrain since the early days of the disaster, but Rheams said high waves and strong easterly and southeasterly winds have complicated the effort.
"The main thing is that they are an indicator that it could be coming more so this way," she said.
State officials have closed a swath of the southern part of the 630-square-mile lake to fishing following the discovery, but there was no sign of impact to wildlife as of Monday, Rheams said.
The Coast Guard reported Sunday that a shift in weather patterns could send more oil toward sensitive shores in Mississippi and Louisiana, and bad weather over the past few days has significantly hampered cleanup efforts. Rheams said that pattern was expected to persist for at least three more days.
Federal officials have estimated that between 35,000 and 60,000 barrels (1.5 million to 2.5 million gallons) of oil have been spewing into the Gulf daily since April 20, when the drilling rig Deepwater Horizon exploded and sank in about 40 miles off the Louisiana coast. Efforts to plug the gusher have so far failed, and oil from the spill has been reported as far east as the central Florida Panhandle.